Carboxylic acids and their anhydrides are widely produced and used in the chemical industry. Streams containing mixtures of a carboxylic acid and its corresponding carboxylic acid anhydride are common. Carboxylic acid anhydrides are used in many esterification processes that result in the formation of the carboxylic acid, so many esterification processes yield carboxylic acid solutions containing small concentrations of the corresponding anhydride of the carboxylic acid. Further, many manufacturing processes that produce carboxylic acid anhydrides result in such streams. Some examples include product refining activities associated with processes for carbonylation of methyl acetate, dimethyl ether, or both, (whether alone or with methanol), as well as processes that involve carbonylation of methyl acetate or dimethyl ether and subsequent reaction of some or all of the resulting acetic acid anhydride with methanol.
Purification of carboxylic acid solutions that contain their corresponding acid anhydrides is challenging because of the difficulty and expense of obtaining pure materials through distillation and because of publications indicating that dilute mixtures of some anhydrides in their corresponding carboxylic acids are significantly more corrosive than either pure carboxylic acid or solutions that contain higher concentrations of the carboxylic acid anhydride in acid. As a result, one designing separation equipment such as a distillation column to fractionate such products would tend to select specialized, corrosion-resistant alloys to address distillation mixtures having more corrosive compositions. However, such a column would involve significantly higher costs due to the costs of such alloys.
Furthermore, sales specifications for some carboxylic acids require that the carboxylic acid be slightly aqueous, that is, that water would be the highest concentration impurity present in the carboxylic acid. Conventional distillation of a mixture of carboxylic acid and the corresponding carboxylic acid anhydride would result in an anhydrous rather than aqueous distillate product because the highest concentration impurity present would be the carboxylic acid anhydride rather than water. Thus, it would be advantageous to convert such streams from anhydrous to aqueous.
It would thus be desirable to develop alternative processes for purification of streams containing carboxylic acids and carboxylic acid anhydrides without using the amount of high-cost, corrosion-resistant alloy required for a distillation column, membrane or other equipment intensive separation process to which produces the substantially pure carboxylic acid. It would also be advantageous to provide a process that yields an aqueous carboxylic acid product.